The invention relates to signal comparison systems. More specifically, it relates to signal comparison systems in which the signals represent visual images for instance. For example, embodiments of the invention may be used to facilitate the comparison of visual signals representing an image which has been produced by a manufacturing process with digital signals representing the desired form which the image should have, the comparison being carried out for the purpose of detecting errors in the image formed by the manufacturing process.
In carrying out such comparison processes, it is clearly important that the two images being compared, or the signals representing them, are accurately aligned. Any misalignment may be misinterpreted as errors in the image under test. Such misalignment can arise as a result of inevitable tolerances in the equipment producing the electrical signals from the manufactured image. Similar errors may result from the digitisation process used to produce the electrical signals representing the images being compared.